In obtaining high strength parts constituting an automobile and the like by press forming work and bending work, a steel sheet used for such work is required to have both excellent strength and ductility. In recent years, in order to make the automobile light in weight and to realize low fuel consumption, it is desired to enhance the strength of the steel sheet used as a material for automobiles, to make the sheet thickness even thinner, and to realize light weight. Also, in order to improve safety performance against a collision of automobiles, further high strengthening is required for structural parts for automobiles such as a pillar and the like, and application of a high strength thin steel sheet with 980 MPa or above tensile strength is under investigation.
As a steel sheet having both high strength and ductility, a TRIP (Transformation Induced Plasticity) steel sheet is being watched. The TRIP steel sheet is a steel sheet wherein austenite structure is retained in steel, and in work deformation at a temperature of martensite deformation starting temperature (Ms point) or above, retained austenite (retained γ) is inductively transformed to martensite due to stress, and thereby large elongation can be obtained. Several kinds of it can be cited, and
(1) a TRIP type composite structure steel with a base phase of polygonal ferrite and including retained austenite (TPF steel),
(2) a TRIP type tempered martensite steel with a base phase of tempered martensite and including retained austenite (TAM steel),
(3) a TRIP type bainite steel with a base phase of bainitic ferrite and including retained austenite (TBF steel),
and the like are exemplarily known.
Out of them, the TBF steel has been known since long time ago, wherein high strength can be easily obtained because of hard bainitic ferrite, fine retained austenite is easily formed in the boundary of lath-like bainitic ferrite, and such structural form brings outstandingly excellent elongation. Also, the TBF steel has a merit in manufacturing that easy manufacturing is possible by one time heat treatment (a continuous annealing step or a plating step).
However, in the high strength region of 980 MPa or above tensile strength, it is known that a harmful effect of delayed fracture due to hydrogen embrittlement newly occurs as the time elapses. Delayed fracture is a phenomenon that, in high strength steel, hydrogen generated from the corrosive environment or atmosphere diffuses into a and a hollow hole in steel and defect portion in a grain boundary or the like, the material is embrittled, stress is applied under this condition, and thereby breakage is caused. The delayed fracture causes a harmful effect such as deterioration of ductility and toughness of metallic materials.
So, the present inventors proposed a TRIP type ultra high strength thin steel sheet with high strength and improved hydrogen embrittlement resistance properties without damaging excellent ductility which is a feature of the TRIP steel sheet in the gazettes of the Japanese Unexamined Patent Application Publication No. 2006-207016, the Japanese Unexamined Patent Application Publication No. 2006-207017, and the Japanese Unexamined Patent Application Publication No. 2006-207018. Here, Mo-added steel added with Mo more preferably by 0.1% or above in order to improve mainly hydrogen embrittlement resistance properties is used.